1. Field of the Invention
The present invention relates to a regeneration method of a working solution in a hydrogen peroxide production process. More specifically, the present invention relates to a regeneration method of a working solution by which various by-products which do not participate in the production of hydrogen peroxide and which are present in the working solution in a hydrogen peroxide production process utilizing an anthraquinone method can efficiently be converted into anthraquinones effective as reaction media for the production of hydrogen peroxide.
2. Description of the Related Art
A main production process of hydrogen peroxide which has currently been practiced on an industrial scale is called an anthraquinone method in which anthraquinones are used as reaction media. In general, the anthraquinones are used by dissolving them in an appropriate organic solvent. This organic solvent may be a single organic solvent or a mixed organic solvent, but it is usually a mixture of two organic solvents. The solution obtained by dissolving the anthraquinones in the organic solvent is called "a working solution".
According to the anthraquinone method, the anthraquinones in the working solution are subjected to reduction (hereinafter referred to as "the hydrogenation") with hydrogen in the presence of a catalyst in a reduction step to produce corresponding anthrahydroquinones. Next, in an oxidation step, the anthrahydroquinones are oxidized with air or an oxygen-containing gas to convert the anthrahydroquinones into the anthraquinones again and to simultaneously produce hydrogen peroxide. Hydrogen peroxide produced in the working solution is usually extracted with water in an extraction step to be separated from the working solution. The working solution from which hydrogen peroxide has been separated is returned to the reduction step again, thereby forming a cyclic process. This cyclic process can produce hydrogen peroxide substantially from hydrogen and air, and hence it is an extremely efficient process. This cyclic process has already been used for the industrial production of hydrogen peroxide.
In the hydrogen peroxide production process utilizing the anthraquinone method, the anthraquinones contained as the reaction media in the working solution are reduced into the anthrahydroquinones, and they are further oxidized to produce the anthraquinones and hydrogen peroxide. While this procedure is repeated, alkyloxyanthrones, alkyltetrahydroanthraquinone epoxides and the like are produced by side reactions. These alkyloxyanthrones, alkyltetrahydroanthraquinone epoxides and the like cannot produce hydrogen peroxide, even when repeatedly subjected to the reduction and oxidation. The production of these useless compounds is very small per occurrence of the reduction and oxidation, but while the circulation is repeated, the above-mentioned compounds are accumulated in the working solution and cause various troubles.
If the nuclei of the alkylanthraquinones are hydrogenated, the alkyltetrahydroanthraquinones are produced, but these alkyltetrahydroanthraquinones have an ability of producing hydrogen peroxide by the repetition of the reduction and oxidation like the alkylanthraquinones. However, the alkyltetrahydroanthraquinones can easily be obtained by reducing the alkylanthraquinones, but the alkyltetrahydroanthraquinones obtained by the reduction have a drawback that their oxidation rate is low. Therefore, as indicated by German Patent No. 2003268, in the case that the alkyltetrahydroanthraquinones are used as the media for the reduction and oxidation, an extremely large energy is required in the oxidation step, and hence half or more of the total energy required in a circulation process is consumed in the oxidation step, which leads to problems regarding apparatus and economy. As one means for solving these problems, for example, in U.S. Pat. No. 5,399,333, ratios of the reduction and oxidation between the alkylanthraquinones and the alkyltetrahydroanthraquinones can be controlled in a suitable range to obtain an economically advantageous process. However, when the alkyltetrahydroanthraquinones gradually increase and the ratios of the reduction and oxidation increase, the above-mentioned problems regarding the apparatus and economy similarly arise.
In view of the above-mentioned problems, in the hydrogen peroxide production process utilizing the anthraquinone method, there is required a step in which the alkyltetrahydroanthraquinones and the alkylanthraquinones are regenerated from compounds such as the alkyltetrahydroanthraquinone epoxides and the alkyloxyanthrones and which cannot produce hydrogen peroxide, and a step in which the alkylanthraquinones are regenerated from the alkyltetrahydroanthraquinones. Thus, many suggestions have been made so far.
In Japanese Patent Publication No. 8806/1964, it has been suggested that inert components are converted into the alkyltetrahydroanthraquinones by treating the working solution with an alkali and an aqueous alkali solution. Furthermore, in Japanese Patent Publication No. 11658/1968, it has been reported that the reduced working solution is treated with caustic soda or sodium silicate at 120.degree. C. to regenerate the alkyltetrahydroanthraquinone epoxides. However, when the working solution in which a part of the anthraquinones are present as the anthrahydroquinones obtained by their reduction is brought into contact with an aqueous alkali solution, the anthrahydroquinones are extracted with the aqueous alkali solution, which leads to an economical problem, i.e., the loss of the expensive anthraquinones.
Japanese Patent Publication No. 19164/1970 has reported that the regeneration of the working solution can be accomplished by treating the working solution with ozone, further treating it with an aqueous caustic soda solution, and then passing it through active alumina at 70 to 75.degree. C. However, this regeneration method comprises 3 steps and it is complicated, and since expensive ozone is used, problems regarding economy and an apparatus are present.
Japanese Patent Publication No. 41040/1974 has suggested a method for regenerating the alkyloxyanthrones by treating the working solution at 130.degree. C. in the presence of a catalyst supporting palladium by the use of an olefin, but a large amount of the olefin and the expensive platinum group metal are used in this method. For these reasons, this method is also considered to be an economically disadvantageous process.
Furthermore, as a method for converting the alkyltetrahydroanthraquinones into the alkylanthraquinones, Japanese Patent Publication No. 4474/1964 has reported that the alkyltetrahydroanthraquinones can be converted into the alkylanthraquinones by bringing alumina, magnesia, a spinel of magnesia-alumina, carbon, or a metal having a hydrogenation ability such as palladium, platinum or nickel into contact with the working solution and a compound having an unsaturated bond such as an olefin. Also in this case, however, in order to heighten a reaction rate, a large amount of the olefin is used and the employment of the expensive platinum group metal is required. Hence, the reported method is also considered to be an economically disadvantageous process.
As understood from the foregoing, the regeneration methods of the working solution by the conventional techniques have some problems regarding an apparatus and economy. For example, a plurality of steps are required and so the operation is intricate, or the addition of a compound other than the catalyst is necessary, or the anthraquinones are probably lost in a certain ratio.
Furthermore, in the conventional techniques, it has not been solved that the alkylanthraquinones and the alkyltetrahydroanthraquinones are regenerated from the alkyloxyanthrones and the alkyltetrahydroanthraquinone epoxides and simultaneously the alkyltetrahydroanthraquinones are converted into the alkylanthraquinones.
On the other hand, the present inventors have found that, in the case that the working solution containing at least the alkyltetrahydroanthraquinones as the anthraquinones is repeatedly reduced and oxidized to produce hydrogen peroxide, alkyltetrahydrooxyanthrones are accumulated in the working solution. The alkyltetrahydrooxyanthrones do not produce hydrogen peroxide, even when repeatedly subjected to the reduction and oxidation, and therefore, the alkyltetrahydrooxyanthrones accumulated in the working solution by repeating the circulation inconveniently cause various troubles. In addition, this fact leads to the loss of the anthraquinones effective to produce hydrogen peroxide, which is economically disadvantageous. The present inventors have found that when the reduced working solution is treated with a catalyst such as alumina with a view to particularly regenerate the alkyltetrahydroanthraquinone epoxides, the concentration of the alkyltetrahydrooxyanthrones increases. For the purpose of solving this problem, in the hydrogen peroxide production process utilizing the anthraquinone method, it is necessary that the alkyltetrahydroanthraquinones should be regenerated from the alkyltetrahydrooxyanthrones.
As the conventional techniques regarding the regeneration method of the working solution, in addition to the above-mentioned techniques, there have been reported a production method of a desired compound from the alkyltetrahydroanthraquinone epoxides in Japanese Patent Publication No. 30801/1982, and a method for regenerating the alkylanthraquinones from the alkyltetrahydroanthraquinones in "Studies in Surface Science and Catalysis", Vol. 88, p. 635 (1994).
In these conventional techniques, however, the regeneration from the alkyltetrahydrooxyanthrones in the working solution has not been described at all.